1. Field of the Invention
This invention relates generally to the electrodeposition of nickel, cobalt, nickel-cobalt, nickel-iron, cobalt-iron or nickel-cobalt-iron. More particularly, it relates to certain N-(sulfopropyl) quaternary ammonium inner salt additives for aqueous electroplating baths that produce bright and level electrodeposits of these metals and alloys.
2. Description of the Prior Art
A recurring problem in the electrodeposition of nickel, cobalt or the above alloys thereof from an aqueous plating bath is the deposition of a metal film having a surface that is dull or discolored in low current density areas. This defect often can be minimized by the addition to the bath of any of a wide variety of water soluble brightening additives, which are described extensively in Modern Electroplating, Third Edition, F. Lowenheim, Ed., pages 287-341, the contents of which are hereby incorporated by reference. Among these brighteners are numerous amino-aliphatic derivatives of oxygenated sulfur-containing acids, examples of which are disclosed in U.S. Pat. Nos. 3,331,868; 3,376,308 and 3,592,943.
Because of the hydrolytic instability in an acid medium of such brighteners in which the aliphatic portion of the molecule is bonded to an oxy or thio atom, these additives exhibit a rapid decline in effectiveness. Brighteners in which both a tertiary amine nitrogen atom and a hexavalent sulfur atom of an oxygenated sulfur-containing acid are directly bonded to an otherwise unsubstituted aliphatic group have, therefore, been favored. A typical brightener of this type is the N-(3-sulfopropyl) quaternary ammonium inner salt, 1-pyridinium-propane-3-sulfobetaine, which is disclosed in U.S. Pat. No. 2,876,177. This compound has been found to be particularly effective when used in combination with ethylene oxide adducts of acetylenic alcohols, as disclosed in U.S. Pat. No. 3,862,019. A major disadvantage of this sulfobetaine plating bath additive is the fact that it is synthesized from the condensation product of pyridine and propane sultone. The latter compound, which is not commercially available and difficult to make, also is considered to be a carcinogen by the Environmental Protection Agency. Although 1-pyridinium-propane-3-sulfobetaine is a very effective nickel brightener, its practical utility is severly limited by the cost of its synthesis and the necessity of removing all unreacted propane sultone.